Besides particle physics, Lee has been active in statistical mechanics, astrophysics, hydrodynamics, many body system, solid state, lattice QCD. In 1983, Lee wrote a paper entitled, "Can Time Be a Discrete Dynamical Variable?"; which led to a series of publications by Lee and collaborators on the formulation of fundamental physics in terms of difference equations, but with exact invariance under continuous groups of translational and rotational transformations. Beginning in 1975, Lee and collaborators established the field of non-topological solitons, which led to his work on soliton stars and black holes throughout the 1980s and 1990s.